Magnetic field imaging is an important technique used in converting magnetic field information to quantitative and visual information. It has wide range of established applications, including nondestructive testing and evaluation (NDE). NDE encompasses damage assessment in metal structures via magnetic materials or induced currents, integrated circuit testing and quality control, general magnetic material and thin film research, permanent magnet quality control, superconductor research and many others. There are several known techniques for visualizing the spatial distribution of magnetic fields or magnetic fields generated by electrical currents. These include for example:                SQUID (superconducting quantum interference device) microscopes,        Magnetic Force Microscopes (“MFMs”),        Magneto-Resistive (MR) scanning sensors and arrays, and        Magneto-optical imagers.        
Of these different techniques, SQUID microscopes probably currently achieve the best magnetic field (or electrical current) resolution and sensitivity. See for example U.S. Pat. No. 5,894,220 to Wellstood et al (April 1999). However, known SQUID microscopes are generally expensive, slow and relatively limited in spatial resolution (e.g., currently limited to about 20-50 μm when imaging objects at room temperature).
MR sensor-based instruments are often cheaper, while offering good magnetic field or current resolution. However, MR sensor-based instruments don't always provide the requisite spatial resolution and speed. MFM offers superior spatial resolution but can be extremely slow, especially when visualizing large areas. In addition, the dynamic range and magnetic field and/or electrical current resolution of magnetic force microscopes are often limited.
Magneto-optical visualizer arrangements are well known. See for example Andrae U.S. Pat. No. 5,583,690; B. Ludescher, et al., “Faraday Low-temperature Microscope for observing Dynamic Magnetization processes in Superconductors (“Faraday-Tieftemperatur-Mikroskop zur Beobachtung dynamischer Magnetisierungsvorgange in Supraeitern”, Laser und Optoelektronik 23 (1991), pages 54-58; L. A. Dorosinskii, et al., Physica C 203 (1992), page 149; and M. V. Indenbohm, et al., Physica C 209 (1993), page 295. Current magneto-optical based visualizers known to those skilled in the art offer high image acquisition speed and good spatial resolution (less than MFM but better than many other techniques). However, in general, such instruments currently have relatively limited magnetic field dynamic range and low-field visualization. They also require magneto-optical films with very specific properties. Such specific properties can be difficult to satisfy at a single temperature and virtually impossible to satisfy over a wide temperature range.
The Kerr microscope for revealing electrical currents using the polar Kerr effect, a reflective technique, is also known. See for example DE 4027049. However, limited polarization rotation can limit magnetic field resolution and dynamic range of the Kerr microscope instrument.
Magneto-optical visualizers that use variations of iron garnet as the magneto-optical material have many advantages. Such a material offers both high polarization rotation, which can translate into high magnetic field or electrical current dynamic range, and a wide range of magnetic properties. Furthermore, the magnetic properties can generally be tuned by adjusting the composition or other parameters. Several iron garnet-based visualizers are known to those skilled in the art.
Michael Faraday discovered magneto-optical (“MO”) effects in 1845. Faraday noticed that magnetic lines of force from a magnet would affect polarized light rays passing through a glass rod. A Scottish scientist named John Kerr later published what came to be known as the Kerr electro-optic effect in 1875. This effect, for which Faraday had searched in vain some 40 years before, is the rotation of the plane of polarization of light in passing through an optical medium across which an electric potential is applied. Kerr's first results were for solid glass, which were followed by results using liquids in transparent cells. In the following year, he published details of another effect, the magneto-optic effect using an electromagnet. The magnetic effect showed that a rotation of the plane of polarization of light occurred on reflection from the polished pole of a magnet.
While the magneto-optical effects observed by Faraday and Kerr in media such as glass were relatively small, these effects are much larger in magnetic media. More recently, MO effects have been used for a variety of applications including magneto-optical recording (e.g., for high density data storage devices), optical communications, magnetic domain imaging, hysteresis loop plotting, Faraday microscopes, and other applications. For example, with the aid of the MO effects dynamic processes in superconductors and magnetic structures in magnetic storage media can be examined. Other applications include, but are not limited to, imaging of electrical current values and distributions on integrated circuits, visualization of magnetization dynamics of spin valves, viewing magnetic inks in currency, non-destructive testing of structural metals and imaging of permanent magnets.
Some magneto-optical visualizers use iron garnet films having perpendicular magnetic anisotropies, i.e., the magnetization vector (M) of the film is directed perpendicular to the film plane in the absence of an applied external field. Such films can be called “perpendicular films”. This type of instrument is disclosed in, for example, U.S. Pat. No. 4,625,167 to G. L. Fitzpatrick (1986), U.S. Pat. No. 5,053,704 to G. L. Fitzpatrick (1991) and U.S. Pat. No. 5,446,378 to S. M. Reich et al. (1995). The general configuration for a magneto-optical imaging film used in such instruments is shown in FIG. 1a. 
As shown in FIG. 1a, a flat wafer, composed of a substrate 2 having an active magneto-optical imaging layer 1 such as a film (MOIF), and a non-magnetic mirror or other high reflectivity layer 3 is placed on or very near the device under test (DUT) 4. The substrates may comprise a material such as Gadolinium/Gallium/Garnet (GGG), or an expanded lattice variant (large lattice constant, LLC GGG), with an applied thin film 1 of a Faraday rotation material, usually with bismuth substituted for yttrium, or Bi:YIG. The active magneto-optical imaging film 1 (MOIF) can be grown on the substrate 2 by, for a nonlimiting example, liquid phase epitaxy (LPE). An incident light 5 from a magneto-optical imaging system is directed to the MOIF structure 1. The reflected light 6 has a polarization state 7. The film 1 renders the magnetic fields visible because the rotation of the polarization of the incident light is magnetic field-dependent (through the magnetic-field dependence of the YIG film's magnetization state), providing intensity contrast when viewed between crossed polarizers.
In perpendicular films, the magnetization in domains is directed either up or down through the film thickness (i.e., always perpendicular to film's surface), thus providing maximum polarization rotation when illuminated in a direction normal to the film direction (see FIG. 1b). However, the perpendicular films can be limited in both spatial and magnetic field resolutions (see FIG. 1c). The spatial resolution is generally limited to the domain size (usually 5 to 50 μm cross-section) and the field resolution is generally limited to 2 bits of information (i.e., there is or is not a field magnitude comparable to or exceeding the coercivity of such a film).
For many applications, MOIF structures based on in-plane iron garnet films (FIG. 2a) are advantageous in terms of both the spatial and magnetic field resolutions. The “in-plane film” is defined as a film that has a magnetization oriented in the plane of the film if no external magnetic filed is applied. Depending on the composition and other parameters of the YIG film, two distinctive cases can be considered (as will be discussed in more detail below): one corresponds to the case of a single easy axis of the film which lies in the plane of the film (it will be denoted as “single-easy axis in-plane” arrangement), while a second corresponds to the case when film has three easy axes in the plane of the film. The latter will be denoted as a cubic-anisotropy, in-plane film, since it is the domination of the cubic crystalline anisotropy that causes this magnetic state of the film). FIG. 2a shows a magneto-optically in-plane active layer 1 disposed on a substrate 2, a high reflectivity layer 3, a device under test (DUT) 4, incident light 5; reflected light 6 and polarization state 7 of the reflected light. When illuminated at normal incidence, such films generally do not exhibit any polarization rotation in the absence of the applied external field. Magnetic fields generated by the DUT (Device Under Test) 4 are generally non-uniform due to domain structure, current flows, magnetic flux patterns, geometry or other reasons. Such non-uniform fields cause local rotation of magnetization vector M from the plane of the film, leading to appearance of the to out-of-plane components of M. Polarization rotation in such a visualizer arrangement will be proportional to the out-of-plane component of M and, thus to the applied perpendicular external field. In such a visualizer, spatial resolution will be greater than that using the perpendicular films because gray-scale information will be obtained from the partial rotation of local vector magnitudes. This increase of spatial resolution will be limited to either the limit of the optical system or to the iron garnet film thickness, which must be large enough to provide enough rotation of the light to obtain a resolvable signal. Such an approach can be used to provide quantitative information on local field strength. See, for example, Nikitenko V. I. et al, IEEE Transactions of Magnetics, Vol. 32, (no. 5) September 1996, p.4639; Valeiko M. V. et al, IEEE Transactions of Magnetics, Vol. 31, (no. 6, pt. 3) November 1995, p.4293; and Nikitenko V. I. et al, Journal of Applied Physics, Vol. 79, (no. 8, pt. 2B), April 1996 p.6073.
Magnetic field and/or electrical current visualizers based on in-plane iron garnet films are seen in, for example, U.S. Pat. No. 5,969,517 to V. R. M. Rao; U.S. Pat. No. 6,084,396 to V. R. M. Rao; U.S. Pat. No. 5,663,652 to M. R. Freeman; and U.S. Pat. No. 6,141,093 to Argyle, et al. However, while much work has been done in the past in this area, further development is possible and desirable.
A simplified schematic view of an exemplary illustrative prior art visualizer for detection of the perpendicular magnetic fields and/or electrical currents is shown in FIG. 2b. In this particular illustrative example, an illumination source P1 provides a light beam that passes through a polarizer P2 to strike a beam splitter P3. The beam splitter P3 directs part of the beam through optics P4 to the MOIF film P5 placed in proximity with a device under test P6. Magnetic fields from the device under test P6 influence the magnet-optical properties of the MOIF film P5—causing localized polarization rotation as is well known. The resulting light with spatially altered polarization distribution passes through optics P4 and is directed via beam splitter P3 through an additional polarizer P7 to a detector P8. In this particular example, the MOIF film P5 comprises an in-plane YIG configuration.
FIG. 2c shows an exemplary image of magnetic fields generated by the surface of a permanent magnet obtained with in-plane YIG and associated visualizer of the type shown in FIG. 2b. By comparing FIGS. 2b and 1c, it is apparent that the information about the magnetic field's value and spatial distribution using an exemplary in-plane YIG is generally much more detailed as compared to the information obtained from an exemplary perpendicular YIG. However, there have been certain disadvantages in using in-plane films in the prior art, although it should be noted that disadvantages are different for different disclosed arrangements.
For example, with regard to the visualizers disclosed by Nikitenko V. I. et al, IEEE Transactions of Magnetics, Vol. 32, (no. 5) September 1996, p.4639, Valeiko M. V. et al, IEEE Transactions of Magnetics, Vol. 31, (no. 6, pt. 3) November 1995, p.4293, Nikitenko V. I. et al, Journal of Applied Physics, Vol. 79, (no. 8, pt. 2B), April 1996 p.6073] and in U.S. Pat. No. 6,141,093 to Argyle, et al., one problem is the difficulty of achieving good enough collimation of polychromatic light from the visualizer's illuminator P1 to provide enough extinction between crossed polarizers. This often results in background noise and a low dynamic range of the visualizer. Narrow band-pass filters can be used to suppress such a problem and to increase the dynamic range by several times. However, due to the low spectral density of commonly available microscope illuminators, CCD camera dark noise becomes an issue if the narrow band-pass filter transmission band is narrow enough (i.e., there is a trade-off between extinction and overall intensity of light reaching the detection unit P8). In addition, variations in microscope illumination intensity and spectral distribution of the emitted light (e.g., due to heating or other effects) may cause a stability problem in this type of visualizer, possibly requiring frequent recalibration and decreasing long-term accuracy.
Use of laser sources for visualizer illumination is known from, for example, U.S. Pat. No. 5,969,517 to V. R. M. Rao, U.S. Pat. No. 6,084,396 to V. R. M. Rao, U.S. Pat. No. 5,663,652 to M. R. Freeman, and U.S. Pat. No. 6,141,09. Laser illumination provides certain advantages but can cause parasitic interference and speckle patterns due to the long coherence length of laser source. These effects may under some circumstances add considerable noise, thus limiting dynamic range and spatial and magnetic field resolution The visualizers disclosed in U.S. Pat. Nos. 5,969,517 and 6,084,396 both issued to V. R. M. Rao appear to avoid this problem by measuring the magnetic field at a single point at a time. However, these arrangements require relatively complicated and expensive mechanics to perform scans over a whole wafer. For example, in such a visualizing instrument, the optics and MOIF film are generally fixed while the wafer is moved on precise mechanical stage. In addition, even though the dynamic range and magnetic field resolution achievable with such visualizers may exceed that of the other prior art visualizer designs, the dynamic range and magnetic field resolution may be limited at low magnetic fields. This is due to the properties of in-plane YIGs, which often have high levels of perpendicular saturation field.
The main driving force in YIG material research and development has been magnetic bubble technology. With the collapse of the magnetic bubble industry, much of the accumulated YIG-growing skills and expertise are beginning to be lost. In the past, thin in-plane YIG films with anisotropy fields (saturation in the perpendicular direction) as small as 3 Oe were not uncommon. See, for example, Vetoshko, P. M. et al., J. of Appl. Phys., 70: (10), pp. 6298-6300, Part 2 November 1991—which along with U.S. Pat. Nos. 5,969,517 and 6,084,396, also discloses single point (i.e., measurement at normal incidence) sensing using such films with balanced detection. Such designs provided the opportunity for visualizing sub-mOe fields. However, the best reported YIGs grown recently appear to have anisotropy fields of at least 40 Oe. See e.g., Klank, M. et al., J. of Appl. Phys., 92 (11), pp. 6484-6488, December 2002. With available films, the magnetic field resolution of the prior art visualizers utilizing in-plane YIGs may be limited to ˜10-100 mOe. Unfortunately, this limits the penetration of such a technique into many high-volume markets, such as IC (integrated circuit) current visualization. Current art MOIF visualizer methods due to the better sensitivity of competitive techniques such as the SQUID microscopy. Therefore, a solution to current art problems is desirable.
Exemplary non-limiting illustrative implementations of the technology herein provides practical magnetic field and/or electrical current visualizing techniques and arrangements offering wide dynamic range, superior spatial and magnetic field (equivalently, electrical current resolutions) and fast image acquisition. A MOIF film with improved imaging properties, and new visualizers that utilize said improved MOIF film are also disclosed.
The designs of exemplary non-limiting visualizers described herein are based on the peculiar magnetic properties of YIG films. For example, even YIG films that are the magnetically hardest in the perpendicular direction (i.e., with high perpendicular saturation fields) often exhibit low in-plane saturation fields (as will be shown below). The in-plane saturation field is often below 10 Oe and sometimes is below 2 Oe. Hence, the visualization of the projection of the external field on the in-plane hard axis of the YIG film can provide sub-mOe magnetic field resolution.
In-plane components of the magnetization vector generally cannot be detected when MOIF is illuminated at a direction that is normal to the YIG film (as for example in prior art visualizers of FIGS. 1a, 2a and 2b) since the polarization rotation occurs only when the magnetization has a component collinear to the light propagation direction. However, detection of the in-plane component is possible when a MOIF film is illuminated at an oblique angle, i.e. the incident beam is tilted by a predetermined angle with respect to the perpendicular direction as schematically shown in FIG. 3a. In FIG. 3a, the “plane of incidence” is the plane containing both the incident and reflected light beams. In one example, in order to achieve linear dependence of the polarization rotation on the value of the projection of in-plane magnetization components on the plane of incidence, the in-plane hard axis of the YIG should lie substantially or entirely in the plane of incidence, while the easy axis may be directed substantially or exactly perpendicular to the plane of incidence. This way, no polarization rotation occurs when no external magnetic field having a non-zero projection on the hard axis direction is present. It should be noted that a YIG film suitable for such a MOIF visualizer might, in one example, have uniaxial energy exceeding by at least an order of magnitude the cubic anisotropy energy. Otherwise, instead of a single easy axis direction in the plane of the YIG film, there may be three easy axis directions.
It should be noted that since the detected magnetization projection should be collinear with the direction of light traveling through the YIG, in the example tilted YIG film visualizer cited, the detected projection of the magnetic field is now also tilted with respect to the light propagation direction. This effect is quite strong since YIGs typically have quite high refractive indices (about 2.3 at a 633 nm wavelength). Thus, in the case of a 45° incidence angle of the beam onto the top surface of the YIG film, the angle of incidence in the YIG will be only ˜17°, providing only ˜33% of the possible signal. Higher tilts can be utilized, but spatial resolution may be sacrificed under certain circumstances. The solution of this problem can be found, for example, by using a prism in contact with the GGG substrate side of a YIG wafer utilizing an index matching liquid between the prism and the GGG. In the case of a right-angle prism, the signal will be 50% of the full signal and for a 60° prism it will be 60%.
Exemplary illustrative non-limiting implementations of the technology herein also provides a practical method of extracting vector information on the spatial distribution of the magnetization in the YIG layer by imaging independently the magneto-optical response in the direction perpendicular to the film and in the direction of tilt. In this case, the in-plane hard axis of the YIG film must be in the plane of incidence of said tilted beam. This can be accomplished by introducing two at least partially separate optical paths. In such an arrangement, the in-plane hard axis and perpendicular magnetization components can be extracted simultaneously. An exemplary method of separation is fairly straightforward. The signal from the first optical path (tilted beam) will take the form Φ1=K11·θ·Mz+K12·θMx, and the signal from the second optical path (perpendicular beam) will take the form Φ2=K2·θMz. In these expressions, θ is the Faraday rotation per unit length of the YIG film; and K11 and K12 are the coefficients describing the sensitivity of the first path to the perpendicular and in-plane magnetization projections. These sensitivity coefficients depend on the particular visualizer implementation. K2 is a similar sensitivity coefficient characterizing the second optical path. A coordinate system is introduced such that the Z-axis is normal to the YIG film and the Y-axis is normal to the plane of incidence. Since the coefficients K11, K12 and K2 are constants during the measurements and can be determined prior to said measurements, the values of the magnetization projections can be obtained according to the simple formulas: Mz=Φ2/(K2·θ) and Mx=Φ1/(K12·θ)−Φ2·K11/(K2·K12·θ). In such a method, a DC bias magnetic field slightly exceeding the coercivity of the YIG film (typically in the range of 0.05-1 Oe) is applied along the Y-axis (i.e. in the direction of the in-plane easy axis). Thus, the YIG crystal will be always be magnetized to saturation, so the magnitude of the magnetization vector can be preliminarily measured. Hence, the third component of the magnetization vector can be also identified according to the formula My==(Mx2−Mx2−Mz2)1/2={Ms2−[Φ2/(K2·θ)]2−[Φ1/(K12·θ)−Φ2·K11/(K2·K12·θ)]2}1/2. This allows full vector information on the YIG magnetization and through it applied magnetic field vector to be determined.
Exemplary illustrative non-limiting implementations also provides a practical method of extracting vector information on the spatial distribution of magnetization in the YIG layer through utilization of a surface plasmon (SP) enhanced MOIF film such as was disclosed in patent application Ser. No. 60/442,539 filed Jan. 27, 2003 entitled “SURFACE CORRUGATION ENHANCED MAGNETO-OPTICAL INDICATOR FILM”, incorporated herein by reference. It is based on the fact that for SP-enhanced MOIF, the MO polarization rotation is maximum around the TM polarization state of the incident light and close to zero around the TE polarization state of incident light. This property allows the use of the TE polarization of incident light for detecting perpendicular external magnetic fields and the TM polarization to detect low-to-very low in-plane fields. In order to do this, the magnetic anisotropy of the YIG should preferably be of the in-plane, single easy axis type and the grating grooves should be collinear to the easy axis direction. In this case, if the polarizer is oriented such that the incident beam has the TM polarization, the reflected beam will have its polarization altered according to both in-plane grooves perpendicular to the grating and perpendicular projections of the YIG film magnetization. If the polarizer is oriented such that the incident beam has a TE polarization, the reflected beam will have its polarization altered according to the perpendicular projection of the YIG film magnetization. The signal processing and control can be similar to that disclosed above (in relation to the non SP-enhanced MOIF films), except that instead of using two separate optical paths, two images corresponding to two states of the polarizer are acquired through a single optical path.